The present invention relates to a process of biologically treating sewage including domestic one. More specifically, the invention relates to a control method in an intermittent aeration activated sludge process which can eliminate nitrogen and phosphorus from sewage.
Conventional processes for treating sewage, which are generally biological processes typically exemplified by the activated sludge process, have been mainly directed to the removal of organic matter. However, in order to cope with the recent serious problem of eutrophication in closed water areas such as lakes and marshes, it is important to eliminate nitrogen and phosphorus that may cause the eutrophication. To this end, as improvements of the activated sludge process, various treatment processes have been developed which can eliminate not only organic matter but also nitrogen and phosphorus. Typical examples of such new processes include the A.sub.2 O process, sequencing batch reactor activated sludge process, and intermittent aeration activated sludge process (hereinafter abbreviated as "intermittent aeration process"). In these processes, bacteria are alternately placed under the aerobic and anaerobic conditions to eliminate organic matter, nitrogen and phosphorus.
The principle of the sewage treatment for eliminating nitrogen and phosphorus is briefly described below. Organic matter in sewage is decomposed and eliminated by being eaten by bacteria that constitute the activated sludge. Nitrogen is eliminated such that NH.sub.4 -N (ammonia nitrogen) is oxidized under an aerobic condition to NO.sub.3 --N (nitrate nitrogen) by an activity of nitrifying bacteria, which is then reduced under an anaerobic condition to N.sub.2 (nitrogen gas) by an activity of denitrifying bacteria. The nitrification and denitrification are summarized below.
______________________________________ Nitrogen form Reaction Reaction change condition Bacteria ______________________________________ Nitrification NH.sub.4 --N .fwdarw. NO.sub.3 --N Aerobic Nitrifying (with DO) bacteria Denitrifica- NO.sub.3 --N .fwdarw. N.sub.2 Anaerobic Denitrify- tion (without DO) ing bacteria ______________________________________
Phosphorus is eliminated by utilizing activated sludge including bacteria capable of storing a large amount of phosphorus in their cells. This type of activated sludge is produced by alternately changing the operation condition of the aeration tank between aerobic and anaerobic conditions. Since this activated sludge releases phosphorus under the anaerobic condition and absorbs phosphorus under the aerobic condition, phosphorus can be eliminated by making the sludge absorb phosphorus under the aerobic condition and then removing the sludge that has absorbed a large amount of phosphorus from the treatment apparatus as excess sludge. The above process is summarized as follows.
______________________________________ Phosphorus concentration Reaction Phosphorus Reaction in tank condition removal ______________________________________ Phosphorus Increase Anaerobic -- release (without DO) Phosphorus Decrease Aerobic Removal of absorption (with DO) activated sludge ______________________________________
As described above, the aerobic and anaerobic conditions are indispensable to the removal of nitrogen and phosphorus. Stated strictly, the anaerobic condition for the denitrification and that for the phosphorus release are different. In the intermittent aeration process, the phosphorus release from the activated sludge occurs after the denitrification is finished and oxygen molecules (originating from NO.sub.3 -N) become absent from the tank, and subsequently the phosphorus absorption is performed in the next aeration step.
Much attention is now given to the intermittent aeration process because the ratio between the anaerobic condition steps can be set in terms of a time period and it can be applied to existing treatment facilities relatively easily. However, in order to efficiently eliminate nitrogen and phosphorus in the intermittent aeration process, it is necessary to properly control the aeration period and the agitation period (anaerobic condition) in accordance with the load. Several control methods have been proposed conventionally, two typical examples of which are disclosed in Japanese Patent Application Examined Publication No. Sho. 63-35317 and Japanese Patent Application Unexamined Publication No. Sho. 64-70198. In the control method disclosed in the former publication, an ORP meter (oxidation-reduction potential meter) is applied to an aeration tank. When the ORP value exceeds the range of +120 to +200 mV, the aeration is stopped to start the agitation. When it becomes smaller than the -250 to -350 mV range, the agitation is stopped to start the aeration. The publication Sho. 64-70198 describes a process for eliminating nitrogen from sewage in which the nitrification and denitrification in a tank is controlled on the basis of a detected ORP changing rate of the tank. More specifically, a bending point of the ORP variation is detected in an aeration step and the aeration step is stopped to transfer to an agitation step with the bending point regarded as a finishing point of the nitrification. In the agitation step for the denitrification, the agitation is stopped to start the aeration step when the ORP changing rate reaches a predetermined value (the denitrification is regarded as finished).
However, since the above control methods are directed to the treatment process that has a single aeration tank and a settling tank, they are associated with a problem that the quality of effluent is not stabilized. To solve this problem, the present inventors have conceived an apparatus consisting of a first aeration tank into which sewage flows, a second aeration tank that is connected in series to the first aeration tank, and a final settling tank, and also have proposed a control method therefor. FIG. 11 schematically shows the main part of this apparatus including a control system. Referring to FIG. 11, an intermittent aeration process and a control method of this apparatus is summarized below. In FIG. 11, paths of water and air are indicated by solid lines and arrows, and control signal lines are indicated by dashed lines and arrows. This apparatus mainly consists of first and second aeration tanks 2a and 2b for eliminating organic matter, nitrogen and phosphorus from sewage 1 flowing thereto by means of activated sludge, a final settling tank 4 for separating the activated sludge by gravitational sedimentation to obtain effluent 3, and a pump 5 for returning the sedimented activated sludge to the first aeration tank 2a. The first and second aeration tanks 2a and 2b have a capacity ratio of 1:1, and the hydraulic retention time of the sewage 1 in the apparatus is 16-32 hours in total. The control system consists of a DO (dissolved oxygen) meter 10a for measuring a dissolved oxygen concentration of the first aeration tank 2a, an ORP meter 6b for measuring an oxidation-reduction potential of the second aeration tank 2b, a control panel 9 for providing, based on the values of the above measurements, control signals to an inverter 11a for controlling the DO concentration of the first aeration tank 2a, a first aeration blower 7a, a second aeration blower 7b, a first agitation pump 8a and a second agitation pump 8b.
According to a typical control method of the above apparatus, the aeration period is set at one hour and the DO concentration of the first aeration tank 2a during the aeration period is controlled at 0.2 mg/1. During the agitation period, an OP changing rate of the second aeration tank 2b is measured and a bending point of the ORP is detected by performing a predetermined calculation. Upon detecting the bending point, the agitation is stopped to start the aeration. The aeration of the first aeration tank 2a and that of the second aeration tank 2b are performed simultaneously, and the agitation of the tank 2a and that of the tank 2b are also done simultaneously.
The treatment process proceeds in the following manner. In the first aeration tank 2a, the nitrification and the denitrification proceed simultaneously (aerobic denitrification) while the DO concentration is controlled to be kept low. In the second aeration tank 2b, the nitrification proceeds while the DO concentration is kept at about 2-3 mg/1 and, at the same time, phosphorus is absorbed by the activated sludge. After a lapse of one hour, the process automatically transfers to the agitation step. In the agitation step, the denitrification finishes in a short period in the first aeration tank 2a because the aerobic denitrification has proceeded in the previous aeration step and therefore the NO.sub.3 -N concentration is low. Then, phosphorus is released from the activated sludge. The denitrification proceeds slowly in the second aeration tank 2b because the organic matter concentration is low and, at the same time, the ORP decreases. Since the ORP varies to produce a bending point when the denitrification is completed, the agitation is stopped when the bending point is detected and the process transfers to the aeration step. Therefore, almost no phosphorus release occurs in the second aeration tank 2b. That is, in the agitation step, the phosphorus release is performed mainly in the first aeration tank 2a and the denitrification is performed mainly in the second aeration tank 2b. By virtue of the existence of two aeration tanks, the above-described process has an advantage that the influent is more unlikely to be discharged without being subjected to the treatment than in the case of using only one aeration tank.
However, the present inventors have thereafter found that the above-described control method has the following problem. That is, since the agitation period depends on the quality of the influent, the efficiency of eliminating nitrogen and phosphorus decreases depending on the concentrations of nitrogen and phosphorus in the influent. For example, where the nitrogen concentration in the influent is low and the phosphorus concentration is high, the agitation period of the second aeration tank 2b is short because the denitrification finishes in a short period. In this case, the short agitation period causes shortage of phosphorus release period in the first aeration tank 2a. As a result, the phosphorus absorption in the aeration step becomes insufficient to deteriorate the phosphorus removal efficiency. Further, since the low DO concentration operation of the first aeration tank 2a suppresses the growth of nitrifying bacteria, the nitrification capability of the entire apparatus becomes insufficient for a nitrogen load when the nitrogen concentration is high. In this case, the nitrification becomes incomplete and the nitrogen removal efficiency becomes low.